Turbine blades and the blade airfoils thereof have long been known from the extensive available prior art. In order that the turbine blades can permanently withstand the high temperatures that arise during operation, they are designed to be coolable. For this purpose, they have, in the interior, a cavity which can be flowed through by a coolant, normally cooling air, during operation. After flowing through the turbine blade and in particular through the blade airfoil thereof, the cooling air, which is heated as it flows through, is discharged into the working fluid of the gas turbine and mixed therewith. If the cooling fluid is cooling air, this is extracted from the compressor associated with the gas turbine. Despite comprehensive measures for keeping the compressor air and also in particular the cooling air clean, said air may still contain dust and dirt particles which, as said air flow through the compressor and also as it flows through the turbine blade, can be deposited therein.
For this reason, modern constructions of turbine blades are inter alia also designed for preventing deposits of such dirt particles at the openings through which the cooling air that is heated during operation is to be discharged. Blockages of such cooling air outlets can have the effect that the cooling action at this location is realized only to a reduced extent, if at all. In this case, the admissible material temperatures are exceeded there, such that the material characteristics consequently change at the overheated location. This permits the formation of local corrosion phenomena and consequential damage, which in the worst case can lead to component failure.
To prevent this, it is known for example from EP 1 793 086 A2 for guide elements to be provided in the interior of the cooling air channels of turbine blades, by means of which guide elements the particles entrained in the cooling air are diverted. This reduces the inflow of the particles into the cooling air outlets.
In an alternative embodiment, known from EP 0 965 728 A2, it is also possible for specially shaped inlets for cooling air openings to be used. Here, by means of an ovalization of the inlet region of the cooling air hole, it is achieved that an entrained particle cannot pass into the hole.
It is a disadvantage that such hole inlets on the inner side of blades that are normally produced by precision casting methods can be produced not by boring but only by casting. Owing to the use of precision casting methods, the cooling air holes however then have a relatively large diameter of at least approximately 2 mm, which undesirably increases the cooling air consumption. Smaller diameters cannot be produced with sufficient accuracy.
It is furthermore known for so-called dust holes to be arranged at the blade tips of turbine rotor blades. Said holes are arranged in the tip region normally centrally between suction side and pressure side and are of relatively large diameter. There is then certainly only a very low risk of blockage, but the cooling air consumption is increased as a result of this. By contrast, if the diameter thereof is reduced, for example in order to save cooling air, there is the risk of blockage, with the stated disadvantages.